HTTP Error when uploading a theme

Hello, I'm experiencing a HTTP ERROR when uploading on one of the themes I set up. We are not sure if its the domain mapping or something else causing it. Have you experienced this issue? I have spoken to the server company WP Engine and they are not sure.

This is the error message.

I get an HTTP Error on the front end upload for [my site]. After talking to hosting, they provided this message from the error log.

XMLHttpRequest cannot load [my site]/wp-admin/async-upload.php. Response for preflight is invalid (redirect)

Would like to know if you have a solution for this issue??